1. Field of Invention
The present disclosure relates to imaging strata that is forward and lateral to an imaging device. More specifically, the present disclosure relates to a logging tool with an electric dipole source and a magnetic sensor that images formation forward and lateral to the tool.
2. Description of Prior Art
A resistivity measurement is one typical subterranean formation evaluation procedure where a log of the resistivity adjacent a wellbore is measured. Formation resistivity is a function of any fluids trapped within the subterranean formation. Thus resistivity is often measured to identify where water and/or hydrocarbon are present in the formation. Changes in resistivity in a subterranean formation can be abrupt and define a bed boundary. Resistivity can be measured with a wireline tool or a logging while drilling (LWD) tool. Measuring resistivity with a galvanic (DC) resistivity device typically involves forming an electrical potential in the formation and measuring a voltage between voltage measuring electrodes of the device. In an induction measurement device, magnetic flux/magnetic field is induced in the formation by the current in the transmitter; which induces a measured voltage in a receiver of the tool spaced axially from the transmitter. During LWD operations, there is a desire to “look ahead” so as to avoid drilling across bed boundaries or faults, as well as any subterranean geological hazard.
Induction-type logging tools have been used to measure formation resistivity as the in-phase response of an induction log is approximately proportional to the formation conductivity. Generally, the in-phase response is much weaker compared to the out-of-phase inductive response, so backing coils are used to suppress the out-of-phase signal. The propagation tool for LWD and geosteering operations, the formation resistivity is measured by the phase difference and the attenuation of responses between a pair of receivers. In geosteering operations, the resistivity measurement is sometimes used to detect a bed boundary and help estimate the distance to the bed boundary. Usually though identifying the bed boundary is a primary concern in geosteering operations rather than obtaining the resistivity measurement. The bed boundary can be detected and estimated more easily by using the stronger out-phase response in an induction type tool, or the response at a single receiver in LWD propagation tool. For example, the cross component response between an orthogonal pair of axial transmitter and transverse receiver can detect the bed boundary around the tool. However, the sensitivity to the distance to the bed in the primary response diminishes once the distance exceeds roughly about the length of transmitter-receiver offset.